In a coating type magnetic recording medium having a magnetic layer mainly comprising ferromagnetic particles and a binder resin, there is an increased demand for higher density recording. Smoothing of the surface of the magnetic layer is a known technique for providing higher density recording.
However, an enhanced surface smoothness of the magnetic layer causes a rise in the coefficient of friction between the magnetic layer of the running magnetic recording medium and the apparatus system in contact therewith. Damage to the magnetic layer or peeling of the magnetic layer can thereby result in a relatively short period of use.
Floppy discs, in particular, are often used under sever high-temperature conditions such as in durability tests. In such cases, the magnetic layers tend to shed ferromagnetic particles, which may clog the magnetic head.
Furthermore, as an effective means for providing higher density magnetic recording media, ferromagnetic metal particles may be employed.
The use of ferromagnetic metal particles in a floppy disc, however, is disadvantageous in that the magnetic layer tends to become damaged due to the rubbing contact of the magnetic layer with the jacket liner to thereby result in dropout, as opposed to the case where iron oxides are used. This problem is often encountered particularly when floppy discs are used under conditions of relatively high temperature such as 50.degree. to 60.degree. C., or when the temperature periodically fluctuates between low and high values.
For improving the running durability of the magnetic layer, it has been proposed to incorporate an aliphatic acid or an ester of an aliphatic acid and aliphatic alcohol as a lubricant in the magnetic layer to reduce the coefficient of friction thereof with the recording apparatus.
For example, it has been proposed to incorporate a monobasic aliphatic acid having 4 or more carbon atoms such as capric acid, lauric acid and oleic acid as a lubricant in the magnetic layer as disclosed in U.S. Pat. No. 3,833,412. It has also been proposed to incorporate an aliphatic ester of a monobasic aliphatic acid having 12 to 16 carbon atoms such as butyl laurate, lauryl palmitate and butyl myristate and aliphatic alcohol having 3 to 12 carbon atoms as a lubricant in the magnetic layer as disclosed in U.S. Pat. No. 3,274,111. It has further been proposed to incorporate tridecyl stearate as a lubricant in the magnetic layer as disclosed in U.S. Pat. No. 4,303,738.
Moreover, it has been proposed to incorporate a higher aliphatic ester of a higher alcohol having a branched molecular structure as a lubricant in the magnetic layer as disclosed in JP-A-55-157131 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Furthermore it has been proposed to incorporate isocetyl stearate as a lubricant in the magnetic layer as disclosed in JP-A-59-186130.
The incorporation of the above described lubricants adversely affects the strength of the magnetic layer, although the use thereof is effective in lowering the coefficient of friction. Therefore, the use of the above described lubricants alone does not prevent the magnetic layer from being damaged by contact with the jacket liner.
Moreover, use of a cellulosic resin as a binder resin having a high modulus of elasticity and excellent mechanical strength to thereby improve the linear-abrasion resistance of the magnetic layer is disclosed, for example, in JP-A-56-13519, 62-239316, 58-70424, 58-70425, 56-74833, 60-133527, 62-34326, 62-26628, 59-79428, 57-135439, 56-74832, and 59-188827.
Binder resins based on conventional cellulosic resins including nitrocellulose, however, do not adequately disperse the ferromagnetic particles, such that not only does the resulting magnetic layer have a poor surface property, but the packing density of the ferromagnetic particles cannot be increased to the desired extent. Consequently, the use of conventional cellulosic resins has been ineffective in providing a magnetic recording media having excellent electromagnetic characteristics.
Furthermore, it has been proposed to use a phenoxy resin as a binder resin as disclosed in JP-A-1-106325, 64-39623, 57-36647, and 57-36648. However, even this approach does not adequately improve the running durability of magnetic layer of a magnetic recording medium comprising a ferromagnetic metal powder with respect to the liner of the jacket of a floppy disc.
Moreover, it has been proposed in U.S. Pat. No. 4,740,418 and JP-B-55-15771 to incorporate an abrasive agent (e.g., hard particles), such as corundum, silicon carbide and chromium oxide in the magnetic layer. The term "JP-B" as used herein means an "examined Japanese patent publication". However, this approach does not protect the magnetic layer from being damaged by the liner of the jacket.